IgE mediates allergic responses by binding to a specific receptor, FceRI, on immune cells. Upon multivalent antigen binding, FceRI receptors become cross-linked, thus initiating activation events. The level of FceRI cell surface expression is controlled by IgE levels. This regulation has great physiological significance since increased FceRI expression translates into enhanced effector function of FceRI expressing cells such as mast cells and basophils. Recent anti-IgE clinical trials have underscored the importance of this regulation by showing that lowering serum IgE results in decreased FceRI expression and decreased basophil effector function. The mechanism of this IgE-mediated FceRI expression regulation has not been well characterized, and we propose to investigate it using the following aims. Aim l: Characterize the process of IgE-mediated FceRI up-regulation. We will determine the mechanism(s) involved. Aim 2: Compare the impact of the two isoforms of FceRI, ag(2) and abg(2) on this regulation. The isoforms are defined by the absence or presence of the b chain, an amplifier of FceRI expression and function. We will follow-up on our preliminary data that IgE-mediated up-regulation of FceRI is significantly greater in ag(2) than in abg(2) expressing cells in vitro. We will determine the molecular mechanism(s) that underlie this difference. We will analyze the physiological significance of this difference in regulation of ag(2) vs. abg(2) in mice. We will determine whether known atopy-associated polymorphisms in the b gene have an impact of IgE-mediated FceRI regulation. Aim 3: elucidate the impact of a newly described product of the b gene on IgE-mediated FceRI regulation. Our preliminary data show that the human b gene gives rise to two alternatively spliced transcripts that are co-expressed and encode two proteins with opposing functions; the classical, already described B, and a newly described truncated protein, bT. We will determine if bT impacts IgE-mediated regulation. We will also assess if the alternative splicing that produces bT is affected by this IgE-mediated regulation. Finally, we will ascertain whether known atopy-associated polymorphisms in the b gene affect the alternative splicing of b. Aim 4: Dissect the minimal structure of FceRI required for IgE-mediated regulation. We will identify the amino acid sequences in the a chain that are necessary to mediate FceRI up-regulation utilizing a similar strategy. Finally, we will verify that the critical sequences thus identified affect the mechanism(s) of IgE-mediated FceRI up-regulation revealed in Aim l. Taken together, these analyses will provide significant insights into the process and molecular parameters that control the IgE-mediated regulation of FceRI expression.